clear all

x = [0  4    8    12   16   20 ]  ;
y = [2 4.93 6.38 6.37 4.88 1.91]' ;

X = [ones(length(x),1) x' x.^2']  ;

a = X \ y

xq = 0:0.1:22 ;
yq = a(1)+a(2)*xq+a(3)*xq.^2 ;

plot (xq , yq) ; axis equal ; grid on ;